The Trump administration’s abrupt decision to slap a $100,000 fee on H-1B visas has stunned and confused employers, students and workers from the United States to India and beyond.

Since announcing the decision Friday, the White House has tried to reassure jittery companies that the fee does not apply to existing visa holders and that their H-1B employees traveling abroad will not be stranded, unable to re-enter the United States without coming up with $100,000. The new policy took effect at 12:01 a.m. Eastern Sunday.

Here’s a look at what the H-1B visa program is and what the Trump administration is doing to it.

Created by the 1990 Immigration Act, they are type of nonimmigrant visa, meant to allow American companies to bring in people with technical skills that are hard to find in the United States. The visas are not intended for people who want to stay permanently. Some eventually do, but only after transitioning to different immigration statuses.

An H-1B allows employers to hire foreign workers who have specialized skills and a bachelor’s degree or the equivalent. They are good for three years and can be extended another three years.

The number of new visas issued annually is capped at 65,000, plus an additional 20,000 for people with a master’s degree or higher. Those visas are handed out by a lottery. Some employers, such as universities and nonprofits, are exempt from the limits.

Commerce Secretary Howard Lutnick said the fee would be applied annually, for a total of $600,000 over the maximum number of renewals allowed. The White House clarified Saturday that it was a one-time fee and said it would not apply to current visa holders.